In communication equipment or AV equipment, an image rejection mixer for attenuating image interfering signals at a frequency symmetrical to a desired frequency, mainly a local signal frequency, has been conventionally employed. Shown in FIG. 6 is an example of conventional receiving device mounted with an image rejection mixer disclosed in Patent document 1.
In FIG. 6, image rejection mixer 2 in conventional receiving device 1 has input terminal 3 to which a signal in the first radio frequency band or a signal in the second radio frequency band narrower than the first radio frequency band is inputted. Also, it comprises first mixer 4 and second mixer 5 directly or indirectly connected to input terminal 3. Further, it includes local signal oscillator 6 for oscillating local signals having phase difference which are applied to these first mixer 4 and second mixer 5. Also, it has filter section 7 for combining a signal from first mixer 4 with a signal from second mixer 5 by rotating at least one of the phase of signal from first mixer 4 and the phase of signal from second mixer 5.
Filter section 7 includes first filter circuit 8 for the first intermediate frequency band which is formed of a polyphase filter, and second filter circuit 9 for the second intermediate frequency band which is formed of a polyphase filter. Also, it comprises first switch 10 for switching and inputting a signal to first filter circuit 8 or second filter circuit 9. Further, it has second switch 11 for switching and outputting a signal from first filter circuit 8 or second filter circuit 9.
Also, receiving device 1 comprises image rejection mixer 2, and intermediate frequency filter 14 for attenuating unnecessary signals that cannot be attenuated by image rejection mixer 2. Further, it includes AD converter 15 connected to the output side of intermediate frequency filter 14, and demodulator 16 connected to the output side of AD converter 15. Also, it has sampling signal oscillator 17 which is connected to the other input terminal of AD converter 15 and produces a sampling signal.
Further, receiving device 1 includes switch controller 12 which obtains channel information and segment information from a channel setter (not shown) in order to control first switch 10 and second switch 11 in accordance with the information. Also, it includes local signal controller 13 which obtains channel information and segment information from a channel setter in order to control the local signal frequency of local signal oscillator 6 in accordance with the information.
In the case of such a conventional receiving device, when a signal in the first radio frequency band or the second radio frequency band is inputted to input terminal 3, switch controller 12 changes over first switch 10 and second switch 11 in accordance with channel information and segment information. And, when a signal in the first radio frequency band is inputted, first filter circuit 8 is selected by first switch 10 and second switch 11. In this way, first filter circuit 8 attenuates image interfering signal existing in the first intermediate frequency band by means of the first attenuation band. Also, when a signal in the second radio frequency band is inputted, second filter circuit 9 is selected by first switch 10 and second switch 11. In this way, second filter circuit 9 attenuates image interfering signal existing in the second intermediate frequency band by means of the second attenuation band.
As described above, for receiving a plurality of desired waves different in bandwidth for example switching and receiving 1-segment broadcast and 3-segment broadcast of ground wave digital broadcast, it is necessary to have first filter circuit 8 and second filter circuit 9 formed of a polyphase filter for each bandwidth. Further, it is necessary to have first switch 10 and second switch 11 for switching first filter circuit 8 and second filter circuit 9. Accordingly, there arises a problem that such a conventional receiving device becomes larger in size.
The present invention is intended to solve the above problem and provide a receiving device which can be reduced in size.
Patent document 1: Unexamined Japanese Patent Publication 2003-298356.